All inks used in ink jet printing today wet the substrate they are deposited on, because wetting is key to adhesion and durability of the finished product. The terms "wetting" and "non-wetting" refer to the appearance of the liquid droplet on the substrate before it evaporates or solidifies Most inks used in ink jet printing today are aqueous based inks containing colorants, which are pigments or dyes dissolved in water based carrier liquids. The term "colorant" is used throughout this application in a generic sense and covers any component of the ink which remains after the carrier liquid evaporates, even if the primary use of the ink in not as a color. An area on a printed surface is classified in the art by the amount of ink which is applied to that area. A "solid" region is completely inked, a "shadow" region is inked over a significant percentage of the printing area with a small percentage remaining un-inked, and finally, a "highlight" region is largely devoid of ink, with inking occurring only on a small percentage of the region.
FIG. 1. shows the prior art, in which a droplet 1 is ejected from a nozzle 2 onto a substrate 3, normally paper. After a few milliseconds of bouncing (FIGS. 1-a, 1-b, 1-c) the droplet starts to wet the substrate as shown in FIG. 1-d. The wetting manifests itself as lowering of the contact angle .o slashed. to well below 90.degree.. The substrate simultaneously absorbs the ink, leaving behind some colorant inside the substrate as the liquid evaporates or solidifies. The dry droplet, FIG. 1-e, shows some colorant absorbed into the substrate. It is also known in the art that heating the printed substrate can increase adhesion and durability of the ink (see for example U.S. Pat. No. 4,308,542).
Wetting is an essential part of all prior art ink jet applications. If the substrate is coated with a material capable of interfering with wetting, the material must be treated to permit wetting. A non-wettable surface may be made porous to promote wetting because increasing a material's surface area increases its surface energy, and increasing surface energy increases wetting. Some materials, which will not wet when applied in a continuous and smooth layer, will wet well when made porous. Porous coatings form the basis of U.S. Pat. No. 5,405,678, which uses a hydrophobic latex to improve the characteristics of a paper substrate. The latex is applied to the paper, but the latex particles on the paper surface are not permitted to coalesce. The significance of leaving the surface porous is clearly stated in U.S. Pat. No. 5,405,678 (column 6, lines 13-30). U.S. Pat. No. 5,405,678 also recommends mixing a strongly hydrophilic material, such as aluminum silicate or activated clay, to promote wetting. Similar ideas of surface coatings which use broken-up or interrupted hydrophobic materials are shown in Japanese patents 1-159287, 64-36478, and 58-132586.
The major disadvantage of wetting is that the ink droplets generate dot sizes which are too large for high quality printing, particularly in the highlight areas of pictures, where ink is often applied one droplet at a time. The problems are augmented when wetting is followed by absorption into the paper fibers, which causes the dot created by the ink droplet to grow in an irregular fashion.
U.S. Pat. No. 5,099,256, issued to Anderson, discloses a technique that substantially reduces wetting of a paper recording medium by ink droplets. Anderson employs an intermediate non-wettable drum surface on which ink droplets are sprayed and then substantially dehydrated before they are transferred to the final paper recording medium. The Anderson invention helps to reduce the amount of wetting on a paper recording medium and thereby reduces the dot size, dot irregularity and color to color bleeding. The need for an intermediate drum with a specialized surface finish is a major drawback. Any wear on the specialized finish will render the invention useless.
Gelbart, a co-owned application, discloses an ink jet printing process for generating smaller, improved definition dots, particularly when using water based inks The process described therein can also be used to generate printing plates for other methods of printing, such as lithographic and flexographic printing plates, and to produce etch resists to act as masks during etching. Gelbart discloses a process wherein a substrate is treated with a thin coating of a hydrophobic material and aqueous liquid ink is applied via a ink jet nozzle to the hydrophobic coating. The hydrophobic coating prevents the ink droplets from wetting the surface and the high surface tension of the water in the ink causes the ink droplets to remain spherical in shape. As the water and other carrier liquids evaporate, a small and very dense dot of colorant remains on the surface. In some cases, the colorant is fused to the substrate using heat to increase durability. Alternatively, a protective overcoat may be applied atop of the printed surface which may serve to maintain the integrity of the non-wetted ink dots.
The process taught by Gelbart works extremely well for highlights, but suffers some shortcomings when printing in solid or shadow areas. Because of the close proximity of the aqueous ink droplets when printing in solid or shadow areas, and the spherical shape of the non-wetted droplets on the hydrophobic surface, the ink droplets tend to coalesce before the aqueous liquids can evaporate. This coalescing phenomenon causes distortion and quality degradation in the printed image.
The principal object of this invention is to provide a process of ink jet printing, wherein the increased resolution and performance described in Gelbart can be maintained in highlight areas without sacrificing printing quality in solid or shadow areas. A second object of the invention is to generate printing plates for other methods of printing (including lithographic and flexographic printing) using the same process. A final object of the invention is to directly deposit chemically resistant coatings, particularly etch resists to be used as masks during etching using the same procedure.